The present invention relates to a protective circuit for a load, and in particular, to a protective circuit for a load DC coupled to the output of a push-pull class AB or class B amplifier powered from a "split voltage " power supply.
Conventional amplifiers utilized in audio systems not using output transformers are often arranged in a "push-pull" amplifier configuration having a pair of transistors in the output stage which are connected in series across a single voltage power supply in what is commonly called a "totem pole" configuration. In such a case, the output signal is supplied to a loudspeaker coupled between the junction of the pair of output transistors and ground through a relatively large coupling capacitor, which for a consumer product, typically is an electrolytic capacitor, which for a consumer product, typically is an electrolytic capacitor. Such an output coupling capacitor prevents the DC voltage at the junction of the output transistors, which is at about one-half the power supply voltage above ground, from being applied to the loudspeaker. However, low frequency audio signals such as bass signals may be substantially attenuated by the coupling capacitor, and thus will not be reproduced by the loudspeaker. Additionally, such a coupling capacitor could be what is commonly called a "non-polarized" electrolytic capacitor in order that the high treble frequency signals are not attenuated by the inductive reactance caused by the internal coil winding of the capacitor plates. Such a "non-polarized" electrolytic capacitor is much more expensive and larger in size for a comparable value of capacitance and voltage rating than a standard "polarized" electrolytic capacitor.
An alternative approach which avoids the disadvantages discussed above with respect to an output coupling capacitor is to use a "split voltage" power supply having equal plus and minus power supply voltages with respect to ground. In this case the output transistors to be adjusted so that the junction point of the transistors is held substantially at DC ground potential. Thus, the loudspeaker can be directly coupled between the junction point of the transistor and ground without requiring an output coupling capacitor since both sides of the loudspeaker are at substantially at the same DC potential of ground and the loudspeaker will not receive any DC voltage. As a consequence, the coupling capacitor can be eliminated and audio signals of very low bass frequencies can be coupled to the loudspeaker substantially unattenuated. That latter feature is of particular importance in higher-power audio systems which typically have larger loudspeakers capable of improved reproduction of the bass frequency signals.
One risk of such a direct-coupling of the loudspeaker to the junction of the output amplifier transistors is that a DC voltage can be applied to the output terminal and to the loudspeaker if a fault occurs in the amplifier. In a typical "single voltage" power supply system, the output coupling capacitor will block such fault producing Dc voltages from the loudspeaker, unless the coupling capacitor itself fails. In any event, such an output DC voltage can damage the loudspeakers.
Some prior art amplifiers provide fuses either between the output transistors and the power supply, or in series with the loudspeaker, or both, to protect against excessive currents in the loudspeakers due to DC voltages. However, fuses are difficult to size properly since they must also carry signal currents, and may "blow" even when a fault condition does not exist. Additionally, even quick acting fuses often will not operate with sufficient speed to protect the output transistors from being damaged.
Another approach is to use a "DC detector" circuit to detect a fault causing a DC voltage to be coupled to the loudspeaker and to shut down the amplifier before damage has occurred. One such a detector is shown in U.S. Pat. No. 4,010,402 of Miyata. Such a circuit provides a low frequency low pass filter to drive a threshold detector for actuating a relay or silicon controlled rectifier (SCR) for respectively disconnecting or shunting the loudspeaker.
Television receivers are often provided with various safety mechanisms. Upon detection of an unsafe condition, a number of courses of action can be taken. If a microprocessor is used, the microprocessor can be programmed to shutdown the primary power supply and to attempt to turn the receive back on a predetermined number of times. If the fault is still detected after the predetermined number of tries, then the microprocessor will keep the receiver turned off. Alternatively, a "safety" circuit an just turn the set "off" and keep it "off" upon detection of a fault. The latter type of circuit is used in the TX-81 chassis of Thomson S.A. of France. Specifically, the chassis is provided with an X-ray protection circuit which is actuated to shutdown the horizontal deflection circuit, and in particular, the horizontal output transistor, upon detection of a fault in a capacitor in the vertical deflection section where such a fault, if allowed to continue, could cause damage. This shutdown of the horizontal deflection circuit shuts down a secondary power supply powered from the flyback transformer. In the TX-81 chassis, the secondary power supply supplies power to at least the vertical deflection section and the audio section. Such a circuit for the TX-81 chassis is disclosed in British Provisional Application No. 8,929,103.3.